media/gpu/chromeos: combine FrameResource Get/CreateNativePixmapDmaBuf

This combines CreateNativePixmapDmaBuf() and GetNativePixmapDmaBuf()
methods. For VideoFrameResource objects, GetNativePixmapDmaBuf() will
create a NativePixmapDmaBuf by duplicating FD's. For instances of type
NativePixmapFrameResource, a reference to `pixmap_` will simply get
returned. Code comments and usage have been adjusted to match the new
class definition.

The FrameResource method HasNativePixmap() and a unit test for the
method have been removed.

Bug: b:277581596
Change-Id: Ie02472e457bcfdf3c4ad627e81b1c5223e5c80c8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5478416
Reviewed-by: Hirokazu Honda <hiroh@chromium.org>
Commit-Queue: Nathan Hebert <nhebert@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1294860}